When a vehicle, such as a dump truck, travels on an uneven ground, a mismatch usually occurs as a result of the displacement of suspensions and tires, and structures constituting a vehicle body (for example, a frame, a vessel, a front axle, a rear axle, and the like) are sometimes put under an excessive load which impairs the durability thereof. When the structures constituting a vehicle body (hereinafter simply called "structures") are continuously subjected to such excessive load, stress accumulates in the structures, and the life of the structures sometimes ends earlier than their estimated life expectancy. For this reason, it takes considerable time to inspect the degree of fatigue in the aforesaid structures when performing regular maintenance.
In order to control the time for maintenance described above, there has been conventionally proposed a device for detecting the load exerted on a frame, for example, a distortion gauge which is directly placed on the frame, or a device for monitoring, for example, by measuring the inner pressure of suspensions which are mounted to a vehicle, and thereby detecting stress.
For example, Japanese Patent Application Publication No. 63-64350 discloses a monitoring device for detecting the inner pressure of gas-filled suspensions, and the explanation thereof will be made below with reference to FIGS. 9 to 11. FIG. 9 shows a schematic view of a frame to explain the monitoring device. As FIG. 9 shows, front axles 42 and 42 are provided at both sides of the front portion of a vehicle, and wheels 6 and 7 are attached via the front axles 42 and 42 to freely travel. A rear axle 43 is provided at a rear portion of the vehicle, and wheels 8 and 9 are attached via the rear axle 43 to freely travel. The front axles 42 and 42 are connected to the front portion of a frame 1 via suspensions 2 and 3, while the rear axle 43 is connected to the rear portion of the frame 1 via the suspensions 4 and 5. A vessel 41 is mounted on the top portion of the frame 1 to freely tilt rearwardly. The frame 1 is supported by the ground via the wheels 6, 7, 8, and 9 and the suspensions 2, 3, 4, and 5. The reaction forces F1, F2, F3, and F4 of the suspensions 2, 3, 4, and 5, respectively, are exerted on the frame 1, and these reaction forces cause stress a, which is expressed by a predetermined functional expression of the reaction forces F1, F2, F3, and F4, and is exerted on the frame 1. Each of the suspensions 2, 3, 4, and 5 is provided with a pressure sensor (not illustrated) for detecting the pressure exerted thereon.
FIG. 10 shows an example of a wave form showing changes in the stress exerted on the frame 1 with respect to time, and FIG. 11 shows frame stress occurrence frequency limit. When a vehicle travels on a so-called uneven ground where the road surface is in a bad condition, the stress .sigma., as shown by a curved line M in FIG. 10, is exerted on the frame 1. A control limit line, which is based on the fatigue resistance of the frame 1 against the stress .sigma., is shown as the limit line L in the chart of the frame stress occurrence frequency limit illustrated in FIG. 11. FIG. 11 shows that a stress having a magnitude of, for example, .sigma..sub.1 can be exerted on the frame 1 N.sub.1 times before an abnormal condition occurs, but that when stress .sigma..sub.1 is exerted thereon (N.sub.1 +1) times, some abnormal condition occurs to the frame 1. Here the stress .sigma..sub.1 represents the upper limit of a normal value of stress exerted on the frame 1, stress .sigma..sub.3 represents a stress close to the threshold value of stress exerted on the frame 1, and .sigma..sub.2 represents an intermediate value between the stresses .sigma..sub.1 and .sigma..sub.3.
In the frame stress monitoring device, configured as described above, each pressure exerted on the suspensions 2, 3, 4, and 5, which is detected by means of a semiconductor sensor (not illustrated), is added at first in accordance with the aforesaid predetermined functional expression, and a stress detecting signal S.sigma. (not illustrated) is outputted. The stress detecting signal is held at each predetermined sampling cycle time, and the held signal is compared with each of the level signals of predetermined strength. The level signals are respectively set as level values corresponding to, for example the upper limit (.sigma..sub.1) of the normal value of stress, stress (.sigma..sub.2) close to the threshold value of stress, and the intermediate stress value (.sigma..sub.3) between the upper limit (.sigma..sub.1) and the threshold value (.sigma..sub.3). Any one of the counters (not illustrated) is operated corresponding to the strength of the stress detecting signal S.sigma., specifically, corresponding to each of the stress ranges .sigma..sub.1 &lt;or.ltoreq..sigma..sub.2 .sigma..sub.1 &lt;.sigma..ltoreq..sigma..sub.3 and .sigma.&gt;.sigma..sub.3, at each of the aforesaid sampling cycle time. Accordingly, an enumerated value of each counter corresponds to the frequency with which stress in the corresponding stress range is exerted on the frame 1. Further, when the enumerated value in at least one of the aforesaid counters equals a previously set value, a light is lit.
As described above, it is detected when the frequency of occurrence of stress reaches the aforesaid control limit line L in FIG. 11, and an operator is informed of the time for maintenance of the frame 1. It should be noted that by using the monitoring device as described above, the operator can be informed of the time for maintenance of each of the other structures in the same way as the frame 1, based on the frequency of occurrence of stress.
As described above, the conventional monitoring device does not inform an operator of the occurrence of a higher load even when a higher load occurs, and it is not until the occurrence frequency reaches the control limit line L that the operator is informed of the occurrence as a warning. Accordingly, with the conventional monitoring device, the next time when passing an area, where a higher load has occurred, preventive measures, or the like, cannot be taken to prevent the occurrence of a higher load. The operator needs to remember the place in order to prevent the occurrence, and a disadvantage is that the higher load cannot be prevented from being exerted on the structures at the place once again if the operator forgets the place or doesn't notice it.
Further, when an operator travels in a new operation site for the first time, the operator has no idea of the road information until he or she actually travels in the site, and there is also a disadvantage in that the operators of the other vehicles operating in the same site cannot obtain information regarding the occurrence of a higher load until they travel in the place.
For this reason, the traveling frequency under high load is increased; therefore, the actual life of the structures is shorter than the expected life, and the number of times maintenance of the vehicle is performed increases. As a result, the cost for maintenance is increased, and the availability of a vehicle is decreased; therefore, it is desired to increase the life of the structures.